Telephone system



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W. A. MALTHANER TELEPHONE 'SYSTEM Filed Feb. 18, 1944 F/G. /4 mx mmm 14 Sheets-Sheet 14 /NVEN TOR WA. MALTHANER ATTORNEY Patented Nov. 6, 1945 UNITEDl STATs PATENT OFFICE TELEPHONE `SYSTEM William A. Malthaner, New York, N. Y., assigner to Bell Telephone Laboratories, Incorporated, New York, N. Yi, a corporation of New York Application February 18, 1944, Serial No. 522,860

13 Claims.

This invention relates to telephone systems and more particularly to small capacity telephone systems of the private branch exchange type (P. B. XJ, in which cross bar switches are ernployed having telephone lines, links and trunks terminating thereon, each of said telephone lines being equipped with an alternating current generating device for producing pulses composed of combinations of diiferent frequencies to designate wanted lines or trunks. A feature of the present invention is a trunk marker available to each of the links upon connection of any one of the latter to a calling line, which marker, in response to a trunk designation registered therein, causes the line to be dis, connected from the link and connected to a trunk. Another feature of the present invention is a calling line checking circuit operable to set a switch in the marker by means of which a memory of the callingline is retained by the marker.

in order that, after the release of the line from the link, the holding mechanism in the cross bar switch individual to the` line may be reoperated to extend said line to the selected trunk.

These andV other features of the invention will* line designation' of 20 and station B having a line` designation of 39;

Figs. 2 and 3 show, diagrammatically, a cross bar 'switch frame of known construction having twenty verticals and ten horizontal outlets.' Tol A each of the vertical outlets a station is connected, station A being connected to vertical 2li, station B to Vertical 39 and the other intermediate eighteen stations, in the order of their respective numerical designations, being connected tover-g' ticals 21 to 38, inclusive. Oi the ten horizontal outlets, the uppermost six are reserved for three links and the lowermost four are reservedfor four trunks, thelatter extending either to one main central 'office (as shown in Fig. 12) or te dinerent central ofliccs;`

Fig. 4 in the left' portion thereof, vshows a link 8.1113171591V ClICllil'), Willie the is't Cf this fg'tll, tO- getner with Fig. 5, shows the call allotter circuit;

Figs.' 6` and '7 showthe details of the link circuit; l

Figs. 3 and '113 snow the link connector and trunk marker connector, respectively;

Figs. 9 and 10 show the common translator and alternating current receiver, respectively;

Fig. 11 shows, in diagrammatic form, two other links and a power generating, tone and alarm circuit;

Fig. 12 shows in detail .one of the four trunks extending between the private branch exchange and the central office, the other three trunks extending to the same central office being conventionally indicated; n

Fig. 14 shows the trunk marker; .while l Fig. 15 vshows the manner in which 1 to 14, inclusive, are to be arranged with respect to each other to completely disclose the invention.

Before describing the detailed operation of the invention it is desirable to call attention to the fact that the invention has been illustrated in connection with the circuits `and equipment completely disclosed and described in Patent 2,300,951 issued to me on November 3, 1941, the equipment and circuits shown in said patent having been modified and adapted to carry out the new functions and features constituting the essence of the present invention, including their adaptation for cooperative action with circuits and equipment not shown therein. Reference, therefore, is made to said patent for an understanding of such operations as are not described herein; for example, the establishment of local connections, that is, connections between the different stations within the private branch exchange itself.

Referring, now, to Fig. 4, there are shown therein three relays, namely, relays CHI, CH2 and CH3, which form a link allotter circuit. Relays CHI and CH2 are shown in the operated position, while relay CH3 is shown in the unoperated position. These three relays control the sequential allotment of an available link to a calling line, and the function of the relays CHI and CH2 in the positions shown is to route the start conductor I to the first available link in the office. As stated before, in the presentembodiment of the invention it is assumed that the volume of traffic from and between the various stations of the ofce, including trunk service to the central manual oince, may be adequately handled by the use of only three links so that the relays CHI, CH2 and CH3 serve to route a calling line to one of the three links furnished depending upon which one of them is free in the order of allotment at the time the line initiates the call. The operated condition of relay CHI indicates that the link to which it providesaccess is in the idlev condition, the relay remaining locked until said link is taken into use, the locking circuit of 2, No. 1 back contacts of relay B of link No. 1 (Fig. 6), conductor 3, No. 3 front contacts of relay CHI to ground. The locking circuit of relay CH2, which controls access to link No. 2, is traced over a path that extends from battery through the winding of said relay, contacts of key KY2, conductor 2 in the bracket line 4, the No. l back contacts of relay B in link No. 2 (said link being conventionally indicated in Fig. 11), conductor 3' in the bracket line 4, No. 6 contacts of relay CH2 to ground.

On the other hand, .the operating circuit of relay CH3, which provides access to link N0. 3, while traceable over the contacts of its associated key KY3, conductor 2" within the bracket line 5, through the No. 1 back contact of relay B of link No. 3 (said link being conventionally indicated in Fig. ll), and conductor 3"' within the bracket line 5, terminates in the No. 3 normal contacts of relay CH3 and, also, in the No. 3 open contacts of relay CHI.

In other words, for a calling line to have ac cessibility to link No. 1 relays CHI and CH2 must be operated and relay VCH3 must be normal. It will be shown that as each link is allotted in rotation, vthe circuits of `relays CHI, CH2 and CH3 are changed to providev for the` advancement of start conductor I from one link to the next in the order designated by the numerical indication of each of the three links.

Bearing in mindthat relays CHI, CH2'are assumed to be operated and that relay CH3 is assumed to be normal, it may now be further assumed that a call is originated at station A (line for a trunk connectionto the manual central oliice. As before stated, the present invention is in the nature of an improvement over the small capacity oflice disclosed in my' abovementioned Patent 2,300,951, in which the stations lthereof are equipped either with a dial device or an alternating current generating device, and in which links are provided whereby local connections between the several lines may be'established over said links by dialing or keying the number I of the wanted station. In the present invention, each of the stations is assumed to be provided with an alternating current device LG and that local connections between the several stations in the office may be established over the links in the manner described in said patent. Although the link circuit herein shown and used has been modified in somerespects over the link in said patent to provide for the perfomance of functions essential yto the present invention, its structure, in so far las it pertains to the` establishment of local connections, remains substantially unaltered. With respect to the establishment of local connections, therefore, theroperations of the link are not described herein, reference being made to the above-mentioned patent for a description of said operations. t

We may now assume that Vit isdesired to establish a connection from station Ato the manual central oflice.` To do so, the attendant at station-A removes the ltelephone instrument from its cradleand then waits for key tone, upon receiving which he keys an arbitrarily assigned digit indicative of the trunk to which connection is desired, say the digit 4. vThe removal of the telephone instrument from the cradle completes a circuit for line relay L20 which extends from ground and battery, over the No. 1 normal contacts of the hold magnet H020 associated with vertical 20, winding of relay L20, ring con- 1 ductor R, over the station loop, tip CQnClllGOl' T,

No. 3 normally made contacts of relay L20 to ground on the No. 2 contacts of cut-off relay C020. Relay L20 operates. Over its No. 3 front contacts, the ground terminal of its circuit is switched from ground on the No. 2 contacts of relay C020 to ground through the secondary winding of transformer LBT to the primary winding of which is connected a source of distinctive tone which, as will be shortly described, is transmitted to the calling line in the event that no link is available.

Over its No. 1 and No. 2 contacts, relay L20 causes a circuit to be completed for relay LO20 in the call allotter circuit (there being one relay LO-therein for each line)Y and relay ST of link No. 1 which, since relays CHI and CH2 of the link allotter are assumed to be operated and relay CH3 is assumed to be normal, is the preferred link in the order of selection. The circuit for the relays ST and LO20 is traced as follows: ground through the No. 1 contacts of relay EI, Winding of relay LO20, conductor 6, No. 1 and No. 2 contacts of relay L20, start conductor I. No. 3 contacts of relay CH2, No. 1 contacts of relay CHI, conductor l, winding of relay ST to battery. Both relays LO20 and ST operate in this circuit. Relay LO20, upon operating, completes a'circuit for relay N, the path of which may be traced from ground on the No. l contacts of relay LO20, conductor I0, No. 1 back contacts of relay M, No. 4 contacts of relay MS, winding of relay N to battery. Relay N operates and completes a circuit for relay M which extends from ground on the No. 2 contacts of relay N, No. 1 contacts of relay MS, winding of relay M to battery. Relay M operates, opens the circuit of relay N at its No. 1 back contacts and causes the latter relay to release. The release of relay N opens the operating circuit of relay .-40 M but this latter relay, however, remains locked to ground on conductor I0 under the control of the No. 1 contacts of relay'MS and its' own No. 2 contacts. When relay MS operates, 'as described below, relay M releases. This cycle lfor operating relays M and N ordinarily performs no useful function, but does perform an emergency function which will be explained in due course. At the same time, relay LO20 completes a circuit for relay E which may be traced from ground on the No. 1 Contact of relay EI, No. 2 contacts of relay LO20, conductor 8, serially through the No. 1 back contacts of the individual link relays LK3, LKZ and LKI in the call allotter, conductor 9, No. 1 back contacts of relay TK, winding of relay E to battery. Upon operating, relay E locks to ground on the No. 1 contacts of relay LO20 and, at the same time, closes a circuit for relays El and E2 extending from ground on its contacts traced thereto from the No. 1 contacts of relay LO20, No. 2 normally made contacts of relay MS, the windings of relays El and E2 in parallel to battery and ground. Relays EI and E2 operate but relay LO20, which operated from ground on the No. 1 contacts of relay EI, now holds locked to ground over its own No. 1 contacts from ground and battery through re'lay ST of the selected links (as previously tracedwto conductors), winding of relay LO20 and its No. 2 contacts, conductor 8 as traced to conductor 9, No. 1 back contacts of relay TK, contacts of relay E, conductor I0, to ground on the No. 1 contacts of relay LO20. Hence, with the operation and locking of relay LO20 individual to the' calling station A, and with the operation of relays EI and E2, all otherlines ation.

may not receiveV service if calls are initiated 'thereat during the time when station A is having pleting the circuit of any other relay LO--in- 'I dividual to a calling line. l

When relay EI operates, it completes a circuit for relay MS, said circuit being traced from ground on conductor I0, No. 14 contacts of relay EI, winding of relay MS tobattery and ground. Since relay MS has slow-operating characteristics, it operates after an interval, at which time it. opens the locking path of relay M, supplies holding ground over its No. 2 contacts for relays EI and E2 and, over its No. 3 contacts, applies ground on conductor I to conductor'I i. Ground on conductor Il is used to complete the circuit of the link select magnet 9 by virtue of operations which will be shortly described and which have been taking place in the link as the result of the operation of relayST therein.

In the event that, due to some trouble condition, relay E of the call allotter fails to operate,

relay MS cannot operate and ground will not be applied to conductor II over its No. 3 contacts. If possible under these circumstances, it

`is desirable to complete the connection, and so emergency means are provided for applying ground to conductor II when relay MS fails to operate. operation of relay L020, a circuit was completed for relay N, and that prior to the operation of relay MS, relay N caused the operation of relay M. Now when relay M operates, it opens the circuit of relay N, but relay M remains lockedf over` its No. 2 contacts and the No. l contacts of relay MS to ground on conductor I0, and it will be observed that, with relay N normal and relay M operated, ground on conductor I0 is applied to conductor Il viathe No. 1 fro-nt con-.-

tacts of relay M and the No. 1 contacts of relay N.'

Returning, now, tothe operation of relay ST of the allotted link No. l, a circuit is completed for select magnet 9, the path of which may be traced from battery, through the winding of said magnet, conductor I1, No. 2 contacts of relay B, No. 4 contacts of relay ST, conductor Il', to the aforetraced ground thereon. Select magnet 9 operates and conditions the group of cross-pointsV Atraced from ground on the No. 3 contacts of relays S and SI, No. 3 contacts of relay LO20,

v conductor I9, winding of relay C020 to grounded battery. Relay C020, upon operating, closes an obvious circuit for hold magnet H020 of the ver-tical to which line 20 (station A) is con-f nected, and because select magnet 9 is .operated cross-point 9 on vertical 20 is also operated, said cross-point 9 remainin-g operated after the circuit of select magnet 9 has been opened, all `in accordance with standard cross bar switch oper- When hold magnet H020 operates, it releases line relay L20 which, in turn, releases relay L020 and the latter, upon releasing. opens the circuit of relay C020 and further discon nects ground from start conductor I which It will be remembered that upon the.A

caused theoperation of relay STL Relay C020 V(as wellaseveryother relay CO-) is slow-release and slow-operate and will hold up for the nonce until a holding circuit. therefor will have been .completed in the link as described hereinafter.

Also, while conductor I is now opened, the fact that cross-point 9 on vertical 20 is closed, relay ST will not release, it having a locking circuit through a cross-point contact, said circuit being traced over .the No. 2 contacts of said relay, No. l contacts of relay lT2, conductor IUI, No, 3 normally made contacts of relay LCi conductor SI, No. 3 contacts of cross-point 9 on vertical '20, resistors RC and RCI to ground.

Since'cross-point 9 on vertical 20 is closed, the line tip and ring conductors T and R., respectively, are joined to .the link tip and ring 'conductors LT and LR, respectively, over the No. 6 and No- 5 contacts, respectively, of lthe operated cross-point 9, and this joining of conductors completes an operating path for relay A of the link, which circuit may be traced from ground and battery, through the lower Winding of said relay, No. 2 back contacts of relay HS, conductor LT, No. 6 contacts of crossepoint 9 on vertical 20, line conductor T, over the calling line loop, ring conductor R, No. 5 contacts of cross-point 9 on vertical 20, conductor LR of the link, No. 3 back contacts of relay HS, upper Winding of relay A to ground. Relay A operates and completesa circuit for relay B which extends from ground on conductor SI, No. 3 and No. 2 normally made continuity contacts on relay LCI', conductor |02, front contacts of relay A, No. 4 normally made continuity contacts of relay B, winding of relay B to grounded battery. Relay B operates and locks over its No. 4 front contacts, conductor IOI, No. 3 normally made continuity contacts on relay LCI', to ground on conductor SI. At its No. 7 contacts, relay B now completes a holding circuit for C020 which traces from battery through the winding of said relay C020, No. 4 and No. 2

contacts in series of cross-point 9 on vertical 20, conductor 5l, No. 'l contacts of relay B, front .r contacts of relay A, conductor |02 and thence as traced to ground on conductor Si. Relay C020 thus holds operated, in turn holding magnet H020 which keeps cross-point 9 closed. At its No. 2 contacts, relay B opens conductor Il, thereby causing the release of select magnet 9. However, since the hold magnet H020 is being held, cross-point 9 on ver-tical 20 will likewise be held.V Relay B further completes a circuit B opens the locking circuit of relay CHI in the allotter circuitvwhich, upon releasing, advances the commonstart conductor I over theHNo. 3 contacts of relay CH2, No. 2 contacts of relay CHI, No.- 1v contacts of relay CH2 toconductor 1 which further extends to relay ST of link No. 2 (conventionally indicated in Fig.- ll).

It is now evident that if link I is busy or is otherwise made unavailable at the time acall is initiated at station A (or any other Station), re'

lay CHI in the link allotter circuit will be normal and the start conductor I, with groundxapplied thereto, will be connected to conductor V1'j of link 2 to'cause theoperation therein of relay ST in the manner described for relay ST of link I. On the other hand, if both links I and 2 are'busy or are otherwise unavailable, relay vCH2 as Well as relay CHI will be normal since their'respective locking circuits will be opened at the No.1 contacts of relay B of their respective links. However, as soon as these relays are both normal,`a circuit is completed for relay CH3 which extends from battery through its winding, contacts of XY3, conductor 2" within bracket line5, No. l contacts of relay B of link 3, (conveniently indicated in Fig. 11') conductor 3" 'to ground over vthe No. 1 normally made contacts of relay CHI.

Relay CH3 operates and, with relays CHI and CH2 normal, represents the condition in which links I and 2 are unavailable but link 3 is available. If, now, a call is initiated at a line station, as for example station A, ground on start conductor I is extended to conductor T" which is connected to the windingfof relay ST Aoflink 3, the path being from ground on conductor I as vapplied thereto from conductor 6 through the No. 2 and No. 3 contacts of relay L20, No. 4 contacts of relay CH2, No. 2 contacts of relay CH3, conductor 1, to the winding of relay ST ofsaid link, causing the operation of said relay and the connection of the associated link 3 to the calling line.

If all three links are unavailable,` relay CH3 will be normal along with relays CHI and'CHZ. The initiation of a call under these circumstances will cause ground on the start conductor I to` be applied to conductor 22 via the No. 4 contacts of relay CH2, No. 1 contacts of relay CH3,'No. 2 contacts of relay CHI, No. 2 contacts of relay CH2, to conductor 22. This conductor extends (within bracket line I3) to the power tone and alarm circuit III) and completes a circuit therein which causes a distinct link busy tone to be applied to conductors 23 and 24 which are connected to primary winding terminals of busy tone transformer LBT. Since the lower terminaI of the secondary winding of this transformer is connected to ground and its upper terminal is completed into a circuit over the winding of the operated line relay L- to battery on the upper normally made contacts of the hold magnet HO-, the subscriber at the calling station will hear the tone as an indication that, due to an unavailable link, the connection cannot be established As soon as a link becomes available, say link I,

' a circuit is completed for relay CHI whichextends from grounded battery through its winding, thence as traced to conductor 3 No. 5 contacts of relay CH2 to ground. Relay CHI operates and locks to ground over its No. 3 front contacts to reestablish the path of start conductor I tothe winding of relay ST of link I in readiness for seizure on a call. If link 2 becomes available, relay CH2 operates over a circuit traceable from grounded battery, through its winding, thence as traced to conductor 3', to ground on the No. 3 normally made contacts of relay CH3.

Returning, now, to the operation of relay ST of link I, assumed to have been taken into use as a result of a call having been initiated at station A, ground on the No. 3 contacts of said relay completes a circuit for relay LSI of its private link connector, and this circuit is traced from ground on said contacts, conductor 58, winding of relay LSI, serially through the No. 2 normally made continuityv contacts of relays LS2 and LS3 (the former. relay being individual to link 2 and the 14 t0 the link.

latter relay individual to link 3), to grounded battery. Relay LSI operates, connects operating battery to its winding through its own No. 2 front contacts and, over its No. 1 front contacts, closes two circuits, the first extending from ground, winding of relay LCI to grounded battery,`and the second extending from the 4same ground, conductor |03, No. 3 contacts of relay DCT, winding of relay LCI' to battery. Both relays LCI and LCI operate, the former to connect the translator and alternating current receiver shown in Figs. 9 and 10, respectively, to the link and the latter to connect'the trunk marker shown in Fig.

In connection with the circuit for operating relay LCI (or LC2 or LC3) ,it should be observed that'the link is designed to control the setting up of two types of connections, namely, a local connection `between two local stations as, for instance,between stations A and B, land a trunk connection `between a local station as, for instance,'station A and a trunk extending to the central oice. 'In the case of a local connection, no trunk is'required and, therefore, the services of the trunk marker (Fig. 14) may be dispensed with whereas, on a trunk connection, its services are required and, therefore, connection of the trunk marker to the link is necessary. Discrimination between the two types of connections is made with the first digit keyed by the calling subscriber. Since in the present embodiment of the invention the private branch exchange is limited to twenty stations with each station having a numerical designation from 20 to 39 depending upon the cross bar vertical in the cross bar switch to which the designated station is connected, and since for the establishment of a local connection the calling subscriber merely keys the two digits of the Wanted local number as set forth in my above-mentioned patent and Whereas for a trunk connection he keys the trunk digit only, itis obvious that if the rst digit keyed by a subscriber is a 2 or 3 and such numerals are not used for trunk digity designations, said numerals 2 and 3 may be used to apprise the link that the connection will be a local one and, therefore, does not require the use of the trunk marker. As described in my above-mentioned patent, the keying of a digit 2 or 3 as the irst digit will cause the operation of relay T2 of the link (if digit 2 is keyed) or relays T2 and T3 simultaneously (if digit 3 is keyed), for registering therein the digit 2 or 3, respectively. Relay T2 completes a circuit for relay CI (as set forth in said patent) and the latter, upon operation, completes a circuit for relay DCT of the trunk marker connector. This circuitmay be traced as follows: Ground and batteri7 through the winding of relay DCT, No. 4 normally made continuity contacts of said relay, conductor m4, serially through the No.6 contacts of the individual trunk relays TKT, TKG, TKS, and TK4 (assuming that four trunks are provided between the private branch exchange and the central office), No. 9 back contacts of relay RTT, conductor |65, No. l contacts of relay LCI conductor IUS, No. 2 contacts of relay CI, No.1. back contacts of relay C, conductor 2l, to ground on No. 6 contacts of relay B. Relay DCT operates over the above circuit, locks over its No. 4 front contacts, conductor I 08, to ground on the No. 5 contacts of relay ON which is operated at this time as will be shown, and opens the circuit of relay LCI' which releases and, in so doing, disconnects the trunk marker from the link.

In the event of a trunk connection, however,

thesubscribermerely. keys the trunk digit designation 4,. Under these circumstances, relayTZ will not be operated and neither will yrelays CI and DCT. `Accordingly, the trunk marker remains connected to the link. The operation of relay LCI transfers the holding circuit'of relay B from ground on conductor SI and conductor IIII as traced thereto over the` normallyl made continuity contacts of relay LCI to the same conductor SI as traced thereto from conductor IOI, No.3 front contacts of relay LCI', conductor 84, No. 1 normally made contacts of relay SA, conductor V8l, No. 4 front contacts of relay LCI' to conductor SI. It also transfers, over its No. 2 frontV contacts, the holding ground for cut-off relay C020 which now holds over conductor III in series with the winding of relay SA to ground on `the No. 3 backi contacts of relay LF. Relay SA now operates over the holding circuit of the cut-off relay C020. 4Over its No. 1 front contacts, relay SA applies ground to conductor 84 to hold relay Band initiates the operation of the trunk marker to check the calling `station as more particularlydescribed hereinafter. Relay. LCI (or relay,LCZ or relay LC3 depending upon which ofthe relays LSI, LS2 or LS3 is operated) also performs certain otherfunctions in the progressive establishment of the connection which are best described when these functions come into play.

Y Relay LCI,v closes a circuit from ground on its No. 5 contacts .over conductor 60 to the winding of relay ON which operates to complete a circuit over its No. 1 contacts to a relayV (not shown) in the time outl circuit I2U, and applies ground overV its Nos. 3 to"5 contacts, inclusive, to the connected conductors for controlling certain relays of the circuit as described hereinafter. With the-operation of relay ON, a circuit is also completedfor relay ONA extending from grounded battery through the winding of said relay, No. 1 contactsn of=relay HR, No. 3 normally made continuity contacts of relay ONA, conductor 16, No. 4 contacts of relay LCI, conductor 15, No. 1 back contacts of-vrelay C, conductor 2I to ground on the No.6 contacts of relay B. Relay ONA operates, locks over its No. 3 front contacts under thecontrol of theNo. vl contacts of relay HR to groundlonconductor 60 and, at the appropriate time performs certain functions which are noted below. The operation of the relay in the time out circuit I20initiates the operation of an appropriate means for measuring a predetermined time interval, which is allowed the calling subscriber to complete keying the Wantedrnumber which, in this case, is the trunk digit. When that interval is terminated, a circuit is completed for relay HR which operates to perform certain functions best described in connection withthe keyingof the assigned trunk digit, in this case 4.

The objectof operating relay LSI and, locally therefrom relays LCI .and LCI is, as before stated, to connect the link when taken into use with the common alternating current pulse receiver and translator shown in Figs. 9 and 10 and with thegtrunk marker shown in Fig. 14. When these circuits are connected to the link the' signal for which is theoperation of relay LS1), the `receiver and the trunk marker vmust be made unavailable to the other two links. This is' done byremoving the operating ground4 for relays LCL-L03; LC2 and LC3 at` the No. 1

back contacts of relay LSI so that if either relayV LSZ or LSiI` should have its operating circuit establishedby relay ST of link 2 or relay ST of link 3, respectively, the circuits of relays LC2 and LC2' or LCS and LC3' will be cut off at the No. 1 back contacts of relay LSI. On the other hand, if the link taken into usehappens to be link 2 or 3, then the corresponding operation of relay LS2 or LSB respectively, will cut off the operating circuit of relay LSI at their respective No. 2 normally made continuity contacts so that if link i is seized thereafter, the failure of relay LSI to operate will prevent the operation of relays LCI and LCI and thereby prevent the connection of the common alternating current receiver, translator and trunk marker simultaneously to two calling links.

The alternating current receiver of Fig, 10 is adapted to receive alternating current impulses from a calling station which, as said before, is equipped with an alternating current generating device LG, 'and this receiver as well as the translator and trunk marker must be connected to the link before the calling subscriber can be advised, by an appropriate warning tone, to transmit the called line or trunk designation. Accordingly, when relay ONA operates, it closes the primary circuit of a, low frequency tone derived from the tone oscillator 'I0 through adjustable resistance pad 62 and transformer 6 I. The path is traced as follows: from the lower terminal of. the secondary (left) winding of transformer 6I, conductor I4No. 1 contacts of relay HM, conductor I4', No. 3 contacts of relay RR, No.2 contacts of relay RA, No. 2 contacts of relay ON, No. 2 contacts of relay ONA, conductor I2, through the low-pass filter 63 to the upper terminal of the secondary (left) winding of transformer E I. The tone frequencies produced by the oscillator I0 are thus induced over the circuit above traced.

Through a transformer arrangement (not shown) in the low-pass filter 63, the low frequen- .ciesare induced in the closed circuit comprising the transformer V68, a transformerrin said lowpass lter 63 and a transformer in high-pass filter 54. Beyond the lsecondary winding of the transformer @not shown) in the filter 64 are connected suitable filter networks (not shown) which are designed to pass only frequencies within the signal frequency band of the frequencies that are produced by the station generator LG when operated.` Hence the tone frequencies produced by oscillator I0 will not pass into the volume limiter $5 or into the enabler 8U. They will, however, be induced into the secondary (upper) winding of transformer 58 if said secondary winding forms apart of a closed electrical system, which it does because conductors 'II and 'I2 are but extensions of the line conductors T and R into the link and alternating current receiver connected thereto. The'path of this circuit is traced as follows: From the left terminal of the secondary (upper) wind ing of transformer 63, conductor 1I, No. 2 back contacts of relay HR, conductor 13, No. 6 front contacts of relay LCI, conductor 54, condenser C2, No, 2 back contacts of relay HS, conductor LTN0. 6 contacts of cross-point 9 on vertical 2U, conductor T, over the loop ofc-station A, conductor R, No. 5 contacts of cross-point 9 on vertical 20, 4conductor LR, No. 3 back contacts of relay HS, condenser C3, conductor 55, No. 7 front con tacts of relay LCI, conductor 14No. 3 back contacts of relay HR, conductor 'I2 to the right terminal ofthe secondary (upper) winding of trans-v -formeri;

Inasmuch as the receiver of the telephoneinstrument of station A is included in the above-closed circuit, the tone induced thereover is heard by the subscriber, which he understands as a signal to proceed with keying the number of lthe wanted local subscriber or of the trunk digit.

If the subscriber failsto key the wanted number or the trunk digit in the time takenfor relay HR to operate as above set forth, it is'presumed that the connection so far set up was due to an accidental removal of the telephone instrument at station A. In any event, the fact that but three links are provided to serve twenty lines and the fact that a link, tied up with presumably a calling line, prevents any other line from getting through due to the use of one common alternating current receiver by al1 three links, makes it imperative that no link be kept unduly out ofservice after key tone is transmitted to the calling line, without said link performing the necessary connection-completing functions that follow the transmission. of the Wanted number designation. Hence if the subscriber fails to transmit the local number or trunk digit by the time relay HR-operates, relay ONA is unlocked,

' a circuiti's completed from ground over the No. 4

contacts of relay HR, No. V4 contacts of relay ONA to a relay (not shown) inthe howler circuit |30 which operates to complete a sharp howler tone which is applied over the No. 2 and No. 3 front contacts of relay HR to conductors 'I3 and 14, bothof said conductors extending over the callingloop (as previously traced) including the station telephone instrument. The tone is suiciently loud to be heard at a distance from the receiver and is intended to apprise the subscriber that 'the telephone instrument should be restored, an occurrence that will cause the link to be disconnected and restored to normal as described hereinafter.

Assuming, however, that relay HR has not operated, the subscriber, upon hearing key tone, operates'the device LG to transmit the wanted designationwvhich, since the connection desired is to a trunk extending to the manual central ofce, will consist of the arbitrary'digit 4 indicating a trunk to said central oice. The subscriber transmits the digit by operating the digit key f4. of the deviceV LG, Now the depression of any key in the device LG causes two out of ve reeds therein t'o be plucked according to a code combination individual to the numeral indicated by the key depressed, and the reedsare set into vibration to produce two alternating` currents of specific frequencies which are then transmitted over a circuit which may be traced as previously to conductors I3 and 14 at the No. 2 and No, 3 back contacts, respectively, of relay HR and, thereafter, to conductors 1| and- 12, respectively, connecting with the secondary.winding of transformer '68. These frequencies are induced into th'e Vcircuit comprising the primary winding of said transformer and the high-pass filter 154.

The alternating current pulse receiver shown in Fig. is a conventional disclosure of the receiver shown in Patent 2,269,022 issued to G. Hecht and A. A. Lundstrom on January 6, 1942. Itcomprises the channel composed of transformer 68, the high-pass filter 54, the volume limiter 65, the transformer 66, the adjustable lpad 51, five detector amplifiers fA-fE with relays FA-FE in the respective anode circuits thereof, `and a suitable filter BPF- intermediate tlie'pad 61 and each one of the detector ampliner tubes fA-fE, each of said lters being designed to pass'only the frequency intended for the particular channel in which the lter is ineluded. Between the high-pass filter 64 and the 2, essi-'45% T volumelimiter 65"is bridged theV primary wind` abler is identical torthefone disclosedjand described inl the above-mentioned? patent to lG. Hecht and A. A. Lundstom and isy shown only conventionally inl Fig."k 10` exceptffor- Vpolarized relay 11. The enabler respondsto a portion of the energy in each of thesign'alapulses that pass through the high-pass'lter |54 and functions to operate vpolarized relay Tl after a predetermined interval, Relay 11 Igrounds conductor 18 which is connected'to the cathode of'each of the de tector tubes fA-fE' thereby activating them' to respond to the portion of the signal 'energy which passes through the Volume limiter`565. j Since'the operation of the-enabler '"llmisl 'completely described in the above-mentioned patent to G. Hecht-and A. A, Lundstrom, Areferencev is made to said patent for a complete 'description thereof, only so'much Vof sait? descriptionbeing supplied herein as will benecessaryto an understanding of the presentinventibn,"

It "11 further ybe ncteqitnatthe'no. 1 contacts f of relay ONA are vbridged acrossthe high-pass inter s4 (through conductors Hs and H1) to short-circuitsaid filter when the relay is normal.4 This is fa- 4safety `measure to insure that if, at the time vthe subscriber initiates the call, the Nos. 5 and 6 contactsof cross-point 9 onl vertical 20 close last, and Vshould close after the alternating current (receiver is connected tothe link, the impulse produced over' the line loop will 'not find a passage into'the 'high-passwfilter. 4For if it should and thepulsewere to contain frequency components within' the 'signal frequency band,

these frequencies would falsely operate one or' more of the "signal channels. wBythel time relay y ONAhasoperatedMhowever, such a pulse will have been dissipated, the short-circuit will haveV been removedfandthe receiver will have been connected to'th 'line as aboye described. i y,

As stated before; each detector amplifier tube fer-fn is preceded by a filter BPF which passeso'ly the frequency intended `for the channelg'in which the tub therein'is to operate on the energycaried by said frequency,Y It is assumed that the device'LG is 'capable of `producing five' different frequencies which, 'for convenience, Inaybe'ds'ignatedlli, inclusivefand that, in accordance 'with theoperationlof the receiver as described in tle'ab'ove-'mentioned patent to G. Hecht and A. A. Lundstrom', frequency A will cause the operationwof tube fA, frequency B will c'aus'e the operation Vof 'tube fB and lsoon, unto and including frequency? E 'which will cause'the operatioh'of tube E." 'Y 'Y As alre'ady"nientioned, eachf detector amplifier tube has a relay F- connected in its anode Ycircuit, anda front contact of this relay is further connected to the winding of one of the ve relays TA-TE in the translator circuit shown in Fig. 9. Relay FA in the anode circuit of the first channel which responds to frequency A has its front contact connected'to the winding of relay TA, while relay FB of the second channel, re' 

